1. Field of the Invention
The present invention relates to silica glass that is substantially free of chlorine (Cl) and has a superior durability against ultraviolet (UV) light, and its method of manufacture.
2. Discussion of the Related Art
Conventionally, reduction projection exposure apparatus called a stepper is often used for exposing and transcribing integrated circuit patterns onto a wafer, such as a silicon wafer, using photolithographic technology.
The optical system of this stepper (reduction projection exposure apparatus) is mainly constructed of an illumination optical system and a projection optical system. The illumination optical system is used, to radiate the light source beam uniformly onto a reticle where an integrated circuit pattern is drawn. The projection optical system is used to reduce the image of the integrated circuit pattern, and project and transfer the reduced image onto the wafer.
Due to recent trends towards greater integration of LSI, it has become necessary for the pattern transcribed on the wafer to have a higher resolution. Therefore, as the light source of the stepper, shorter wavelengths, from the g-line (436 nm) and the i-line (365 nm) to excimer lasers, such as KrF (248 nm) and ArF (193 nm), are increasingly being used.
Conventional optical glass used as the lens for these illumination optical systems and projection optical systems has a problem of its low light transmittance in the short wavelength range below the i-line. Currently, synthesized silica glass is used as the lens material instead of conventional optical glass.
This silica glass can be manufactured (synthesized) for example, by vapor-phase synthesis called the "direct method". This direct method include the following steps, for example.
(1) The step for expelling a gaseous silicon compound as a material, an oxygen gas, and a hydrogen gas from a burner made of silica glass. In general, the gaseous silicon compound is diluted with a carrier gas (an oxygen gas, for example) upon emission.
(2) The step for generating minute silica glass particles (soot) by reacting the gaseous silicon compound and water which is a reaction product of the oxygen and hydrogen gases.
(3) The step for depositing the minute silica glass particles on a target.
(4) The step for creating silica glass (lump) by fusing and vitrifying the minute silica glass particles deposited on the target.
It has been discovered that the silica glass manufactured with this manufacturing method using silicon tetrafluoride gas as a gaseous silicon compound is substantially free of chlorine in the silica glass and exhibits a high durability against ultraviolet light, as compared with silica glass manufactured using silicon tetrachloride as the material.
However, when silica glass is synthesized using the silicon tetrafluoride (SiF.sub.4) gas as the material, a new problem emerges as a trade-off for the high durability to ultraviolet light. The problem is that the uniformity in refractive index tends to deteriorate in the resultant silica glass.